Correcting spatial beam deformation

ABSTRACT

The invention disclosed here teaches methods and apparatus for altering the temporal and spatial shape of an optical pulse. The methods correct for the spatial beam deformation caused by the intrinsic DC index gradient in a volume holographic chirped reflective grating (VHCRG). The first set of methods involves a mechanical mean of pre-deforming the VHCRG so that the combination of the deflection caused by the DC index gradient is compensated by the mechanical deformation of the VHCRG. The second set of methods involves compensating the angular deflection caused by the DC index gradient by retracing the diffracted beam back onto itself and by re-diffracting from the same VHCRG. Apparatus for temporally stretching, amplifying and temporally compressing light pulses are disclosed that rely on the methods above.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation and claims the prioritybenefit of U.S. patent application Ser. No. 12/460,060 filed Jul. 13,2009, which claims the priority benefit of U.S. provisional patentapplication No. 61/197,458 filed Oct. 27, 2008, the aforementioneddisclosures being incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for altering thetemporal and spatial shape of an optical pulse. Pulse stretchers basedon volume holographic chirped reflection gratings (VHCRG) are used forincreasing the temporal length of an optical pulse prior toamplification by an optical amplifier. After amplification, the opticalpulse is temporally recompressed by a pulse compressor in order toachieve a short duration pulse. During the process of stretching andcompressing, the spatial shape of the pulse can be distorted by thevolume grating. It is desirable to obtain a mean to produce a beamspatial profile that is clean, i.e. free of spatial distortion after thestretching and compression steps by diffraction from a chirpedreflecting volume holographic grating.

Portions of the disclosure of this patent document contain material thatis subject to copyright protection. The copyright owner has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure as it appears in the Patent and Trademark Office fileor records, but otherwise reserves all copyright rights whatsoever.

2. Background Art

FIG. 1 illustrates a state-of-the-art pulse stretcher/compressor pairthat produces a high power short pulse. A seed oscillator optical pulse100 is collimated and directed to a pulse stretcher comprised of twodispersive diffraction gratings 110 and a pair of lenses positioned inbetween. The diffraction gratings 110 are placed one focal length awayfrom the lenses. The stretched pulse 120 is amplified by an opticalamplifier 130, whose output produces a high power stretched pulse 140.The high power long pulse is shortened by a compressor that uses twodispersive diffraction gratings 160. The output of the compressor is ashort and intense pulse 160.

The compressor/stretcher based on dispersive grating are bulky due tothe small angular dispersion that can be achieved. In contrast, a pulsestretcher/compressor based on non-dispersive volume holographic chirpedreflection gratings (VHCRG) is several times smaller. FIG. 2 illustratesthe concept. A seed oscillator optical pulse 200 is collimated anddirected to a pulse stretcher that is comprised of a VHCRG. The inputaperture is typically several square millimeters. The VHCRG can be madeout of different thick holographic materials such as photo-thermal glass(PTR) or crystals which have a high peak power damage threshold.Commercial PTR VHCRG typically have several hundreds of MW/cm² damagethreshold for 20 ns pulses at 20 Hz repetition rate near 1 μm. FIG. 3illustrates a damage threshold measurement for commercial PTR volumeholographic material.

In PTR holographic glass, a small DC index change arises between the topand bottom of the VHCRG. Absorption of the recording beam during therecording process creates an uneven exposure in the direction of therecording beam throughout the thickness of the material. In holographicphoto-thermo refractive glass for example, this exposure change createsa small DC index change of the order of 10⁻⁴.

The DC index change is related to the illumination exposure and thusalong the thickness of the sample, the DC index change variescontinuously. The DC index gradient affects the propagation of acollimated beam. FIG. 4 illustrates this effect. An undistortedcollimated beam 400 with a beam size of the order of the thickness ofthe VHCRG 410 will be diffracted into beam 420 in the direction of theDC index gradient thus deforming the spatial profile of the incidentbeam. The output beam profile 430 is shown in FIG. 4. The extent of theangular deflection can be approximated by the following formula:α≈(∂n/∂z) L/n, where a is the deflection angle, (∂n/∂z) the index ofrefraction gradient, L the length of the VHCRG and n its average indexof refraction. For example, the expected deflection angle in the case ofan index gradient of 10⁻⁴/mm, length L of 30 mm and average index of 1.5yields a deflection angle of 2 mrad. Because the diffracted beampropagates twice the length L of the VHCRG (by reflection), the totaldeflection angle becomes 4 mrad. After a free space propagation of only25 cm, a 1 mm diameter pulse diffracted by the VHCRG will be elongatedin one direction (the direction of the DC index gradient) by 1 mm. Theextent of the oblong spatial beam profile of the diffracted beam 430matches the above quantitative explanation. Although small, the effecton the spatial beam profile is detrimental for proper amplification ofthe stretched pulse. It is also detrimental when the recompressed pulseneeds to be close to distortion free for applications such as but notlimited to thin film photovoltaic scribing, precise machining andablation.

In order to increase the time delay, while maintaining the same lengthVHCRG, a double pass configuration with a VHCRG is used. FIG. 5illustrates the method. A seed oscillator optical pulse 500 iscollimated and directed to a pulse stretcher that is comprised of aVHCRG 510 and a flat mirror 520. The angular positioning of the mirroris such that the diffracted beam is reflected and counter propagating.The double pass in the VHCRG 510 increases the time delay by a factor 2with respect to the single pass configuration illustrated in FIG. 2.However, the beam distortion is amplified by a factor 2 as well. FIG. 6illustrates this effect. The incident beam is diffracted by the VHCRG600 and reflected by a flat mirror 610 to produce a counter-propagatingbeam which is in turn re-diffracted by the VHCRG 600 to produce beam620. At each diffraction, the deflection increase towards the DC indexgradient.

SUMMARY OF THE INVENTION

A method is proposed to correct for the spatial beam deformation causedby the intrinsic DC index gradient in a VHCRG.

The second set of methods involves a mechanical mean of pre-deformingthe VHCRG so that the combination of the deflection caused by the DCindex gradient is compensated by the mechanical deformation of theVHCRG. The first set of methods involves compensating the angulardeflection caused by the DC index gradient by retracing the diffractedbeam back onto itself and by re-diffracting from the same VHCRG.Apparatus for temporally stretching, amplifying and temporallycompressing light pulses are disclosed that rely on the methods above.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims and accompanying drawings where:

FIG. 1 (prior art): pulse stretcher/compressor with dispersivediffraction grating.

FIG. 2 (prior art): pulse stretcher/compressor with non-dispersivevolume holographic chirped reflective grating (VHCRG).

FIG. 3 (prior art): damage threshold measurement for a volumeholographic photo-thermal glass.

FIG. 4 (prior art): illustration of the spatial beam distortion createdby a DC gradient index in a photo-thermal volume holographic chirpedreflective grating (VHCRG).

FIG. 5 (prior art): pulse stretcher/compressor with non-dispersivevolume holographic chirped reflective grating (VHCRG) with double passarrangement.

FIG. 6 (prior art): details of the double pass arrangement of FIG. 5.

FIG. 7: illustration of a compensated double pass arrangement with VHCRGto provide a distortion free diffracted beam.

FIG. 8: beam profile measurement of the diffracted beam using the methodof FIG. 7.

FIG. 9: pulse stretcher/compressor apparatus with non-dispersive volumeholographic chirped reflective grating (VHCRG) with double passarrangement method of FIG. 7.

FIG. 10: illustration of a mechanical mean to pre-distort the VHCRG toprovide a distortion free diffracted beam.

FIG. 11: three-dimensional rendition of the illustration in FIG. 10.

FIG. 12: Temperature dependence of the beam profile using the package ofFIG. 10.

FIG. 13: detailed measurement of the beam profile at fine temperatureincrement using the package of FIG. 10.

FIG. 14: Spectral measurement of the VHCRG.

FIG. 15: illustration of an apparatus with uniform beam profile afterpulse stretcher/amplification/compressor with two VHCRGs packagedaccording to FIG. 11.

FIG. 16: illustration of an apparatus with uniform beam profile afterpulse stretcher/amplification/compressor with a single VHCRG packagedaccording to FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the present invention, reference is madeto the accompanying drawings which form a part hereof, and in which isshown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

FIG. 7 illustrates the method. A right angle mirror or right angle prism710 replaces the flat mirror found in FIG. 6. The right angle mirror orright angle prism 710 retraces the diffracted beam 720 back onto itselfto produce beam 730. During the first diffraction by the VHCRG 700, thebeam is no longer collimated in the direction of the DC gradient.However, because the right angle mirror or right angle prism 710reflects the diffracted beam 720 back onto itself irrespective of thecollimation in the direction of the gradient index, the seconddiffraction recollimates the beam to provide an undistorted beamprofile.

FIG. 8 shows the spatial profile resulting from using the method of FIG.7. For the compressor, the orientation of the VHCRG is reversed withrespect to input beam. The same right angle mirror or prism arrangementis used.

Another embodiment in the invention is the apparatus of FIG. 9 which isuses the embodiment above illustrated in FIG. 7 to provide a spatiallyclean beam after temporally stretching, amplifying and re-compressingthe pulse.

A seed oscillator optical pulse 900 is collimated and directed to apulse stretcher that is comprised of a VHCRG 910 and a right angle prismor right angle mirror 920. The distortion-free temporally stretchedpulse 930 is amplified by an optical amplifier medium 940 which can be,but not restricted to a fiber amplifier or a free space amplifier. Theamplified beam 950 is fed into a pulse compressor that is comprised of aVHCRG 960 and a right angle prism or right angle mirror 920. The VHCRG960 is a stretcher used in reverse i.e the chirp direction is reversed.A right angle prism or right angle mirror 970 is used as well to correctfor the spatial distortion. This can be realized for example by cuttinga VHCRG in two pieces and using one piece as a stretcher and the otheras a compressor. The imperfection in the fabrication of the VHCRGstretcher such as the non-linearity of the chirp rate or chirp amplitudecan then be corrected by the compressor with near identicalimperfections. Beam 980 is a high power short pulse after temporalcompression by the VHCRG compressor.

In another embodiment, a VHCRG 1010 is mechanically deformed by applyingpressure on one or more points while the edges of the entrance and exitfacets 1040 and 1050 of the VHCRG 1010 respectively are resting on amount 1020. In general, any mechanical apparatus that provides bendingin a direction approximately orthogonal to the incident light direction1025 and in the direction of the gradient can be used. FIG. 10 shows anexample only. A screw 1030 provides an adjustable mean for varying thepressure on the VHCRG and thus the amount of bending. The dimension ofthe mount 1020 may vary with the cross section and length of the VHCRG1010. In general, consideration must be adequately taken to provideenough stiffness in the mount to enable bending the VHCRG.Experimentally, the incident distortion-free beam profile 1000 isdiffracted by the VHCRG to produce a distortion-free stretched beam1060. Due to the symmetry of the device, the compressor also produces adistortion-free beam. A three-dimensional rendition of the mountrealized with the VHCRG 1100 in a mount 1120. A screw 1110 positionedapproximately, but not restricted to, the middle of the mount 1120, canadjust the amount of stress (bending) applied to the VHCRG 1100. Thepackaged VHCRG of FIG. 11 has been tested a different temperature. Thebeam quality a three temperature, 11° C. (1200), 25° C. (1210) and 38°C. (1220) is shown respectively in FIG. 12. FIG. 13 shows more detailedmeasurement of the spatial beam width in two axis at finer temperatureincrements. The good beam quality of the temporally stretched,compressed beam using the packaged VHCRG of FIG. 11 is also demonstratedin FIG. 14. A lens 1400 collimates the output of a single mode fiber(not shown). The light source is a wide spectral band source (40 nmFWHM). The collimated beam 1410 is diffracted by the packaged VHCRG1430. The diffracted beam 1440 has a spectral width which is equal tothe spectral width of the VHCRG. A beam splitter 1420 picks off thediffracted beam 1440 and redirects it to a lens 1450 which focuses thelight into a single mode fiber 1460. The output of the fiber 1460 is fedinto a spectrometer 1470. The spectrum 1490 of the diffracted beammatches the spectral bandwidth of the VHCRG. The achieved couplingefficiency of 70% proves that the beam quality is near distortion-free.

Another embodiment in the invention is the apparatus of FIG. 15 which isuses the embodiment above illustrated in FIG. 10-14 to provide aspatially distortion-free beam after temporally stretching, amplifyingand temporally re-compressing a pulse. A seed oscillator optical pulse1500 is collimated and directed to a pulse stretcher that is comprisedof a packaged VHCRG 1510 according to embodiments disclosed in FIGS. 10and 11. The distortion-free temporally stretched pulse 1520 is amplifiedby an optical amplifier medium 1530 which can be, but not restricted toa fiber amplifier or a free space amplifier. The amplified beam 1540 isfed into a pulse compressor that is comprised of a packaged VHCRG 1550,according to embodiments disclosed in FIGS. 10 and 11. The packagedVHCRG 1550 is a stretcher used in reverse i.e the chirp direction isreversed with respect to the stretcher. This can be realized for exampleby cutting a VHCRG in two pieces and using one piece as a stretcher andthe other as a compressor. The imperfection in the fabrication of theVHCRG stretcher such as the non-linearity of the chirp rate or chirpamplitude can then be corrected by the compressor with near identicalimperfections. Beam 1560 is a high power short pulse after temporalcompression by the VHCRG compressor.

In yet another embodiment, A seed oscillator optical pulse 1600 iscollimated and directed to a pulse stretcher that is comprised of apackaged VHCRG 1610 according to embodiments disclosed in FIGS. 10 and11. The distortion-free temporally stretched pulse 1620 is amplified byan optical amplifier medium 1630 which can be, but not restricted to afiber amplifier or a free space amplifier. The amplified beam 1640 isdirected by a set of mirrors towards the opposite facet of the sameVHCRG 1610. The temporally stretched beam 1640 is temporally compressedby the VHCRG 1610 to produce a high power short pulse beam 1650.

In all the embodiments above, the optical radiation whose temporal andspatial profile is altered can be produced, but not limited to, asemi-conductor laser, a solid state laser, a fiber laser in the range of266 nm to 2.5 micrometers.

1. A method for correcting spatial beam deformation, the methodcomprising: resting a volume holographic chirped reflective grating on amount, wherein an optical beam propagated in free space and directed atthe volume holographic chirped reflective grating is temporallystretched and wherein the volume holographic chirped reflective gratingis determined to cause spatial beam deformation of the optical beam; andapplying mechanical pressure to the mounted volume holographic chirpedreflective grating, wherein the application of mechanical pressurecauses bending of the volume holographic chirped reflective grating andwherein the bending corrects the spatial beam deformation of the opticalbeam.
 2. The method of claim 1, further comprising adjusting themechanical pressure applied to the mounted volume holographic chirpedgrating, the adjustment of the mechanical pressure corresponding to anamount of bending.
 3. The method of claim 1, further comprising testingthe volume holographic chirped reflective grating by measuring qualityof the optical beam diffracted by the volume holographic chirpedreflective grating over a range of temperatures.
 4. The method of claim1, wherein the volume holographic chirped grating is made out of photothermal glass and the mechanical pressure is applied using a means forthermal binding.
 5. An apparatus for correcting spatial beamdeformation, the method comprising: a mount for holding a volumeholographic chirped reflective grating, wherein an optical beampropagated in free space and directed at the volume holographic chirpedreflective grating is temporally stretched and wherein the volumeholographic chirped reflective grating is determined to cause spatialbeam deformation of the optical beam; and a means for applyingmechanical pressure to the mounted volume holographic chirped reflectivegrating, wherein the application of mechanical pressure causes bendingof the volume holographic chirped reflective grating such that thebending corrects the spatial beam deformation of the optical beam. 6.The apparatus of claim 5, wherein the means for applying mechanicalpressure is further used to adjust the mechanical pressure applied tothe mounted volume holographic chirped grating, the adjustment of themechanical pressure corresponding to an amount of bending.
 7. Theapparatus of claim 5, further comprising: an amplifying means forincreasing power of the corrected optical beam to produce an amplifiedoptical beam; and a compressing means for temporally compressing theamplified optical beam, the compressing means including a second volumeholographic chirped reflective grating under a same amount of mechanicalpressure as the volume holographic chirped reflective grating.
 8. Theapparatus of claim 7, wherein the second volume holographic chirpedreflective grating is fabricated from a common piece as the volumeholographic chirped reflective grating.